1. Field of the Invention
The present invention relates to a radial tire improved in steering stability, durability and appearance by adopting a specified nylon cord of high modulus and excellent dimensional stability.
2. Description of the Prior Art
A nylon cord is widely used in bias tire for its excellent fatigue resistance and adhesive property to rubber. However, nylon cord heretofore, has not been used in a radial tire, because of the following problems.
When driving an automobile at high speed or under a heavy load, it is often the case that the temperature of the carcass of a tire rises up to 80.degree..about.120.degree. C. depending on size of the tire. In a known cord composed of nylon 6 or nylon 66, loss tangent (tan .delta.) of dynamic viscoelasticity at the aforementioned temperature is so large that a flat spot is easily occurs after cooling.
Furthermore, when turning the automobile by reorienting tires, the automobile still has a tendency to maintain the running course as before due to inertia and produces a centrifugal force in the direction opposite to the turning direction of the automobile. As a result, a force to draw the automobile in a course of the tires, i.e., a side force acts from the road on the tires against the centrifugal force. In the case of conventional nylon cord, the modulus declines quite an extent when the temperature reaches 100.degree. C. and over. Accordingly, when a tire composed of conventional cord is used at high speed and the temperature thereof rises up to such a degree, the rigidity of the carcass of the tire is inclined, and when the side force is added at the time of turning, the tire becomes deformed to reduce the side force, eventually declining the controllability thereby.
Moreover, a manufacturing process of these tires usually comprises the step of applying a rubber coating to a tire cord fabric composed of a plurality of cords, the step of cutting the rubber coated tire cord fabric at the specified length, and a step of lap-splicing both ends of 2-10 pieces of fabric along cord to obtain a cylindrical green tire. Thus it is inevitable for the tire to have at least a lap-splice portion. Since the density of cord at such a lap-splice portion is necessarily high as compared with other portions of the tire, at the vulcanization step, the heat shrinkage factor of the lap-splice portion in its entirety becomes larger even if the heat shrinkage stress of each cord remains unchanged. As a result, a side wall undulation occurs in the side of the tire affecting the appearance and declining the modulus of the cords of the tire.
As discussed above, when the temperature of the tire carcass is higher than 100.degree. C. under high speed driving of an automobile, the modulus of the conventional nylon 6 and nylon 66 cords is small and the thermal dimensional stability thereof is low.
To meet these problems, a radial tire for use in passenger cars comprising a nylon cord of relatively small heat shrinkage composed of nylon 6 or 66 having been spun at a high speed was proposed as disclosed in Japanese laid open Patent Publication (unexamined) No. 59-89203. This nylon cord of low heat shrinkage has a thermal dimensional stability approximate to that of polyester, which is not sufficient and the modulus at high temperature is still low. As a result, steering stability of the proposed radial tire is insufficient, and moreover there exists such problems, as occurrence of flat spot, poor appearance due to side wall undulation in the tire, etc.